System for starting induction motors with self-excitation

ABSTRACT

A polyphase induction machine has a balanced polyphase shunt capacitor bank connected to the stator. In starting as a motor, the stator and capacitor bank are supplied with reduced line voltage by means of a transformer, and the motor is allowed to accelerate until it reaches maximum (and nearly) synchronous speed. Then, the motor is disconnected from the source and permitted to coast, whereupon there is a building up of stator voltage by self-excitation. When the phase and amplitude of the self-excited voltage are approximately equal to the phase and amplitude of the line voltage, the stator is connected directly to the line and the transformer and excitation capacitor are disconnected. The starting is accomplished with minimal dip in line voltage.

United States Patent Robb [54] SYSTEM FOR STARTING INDUCTION MOTORS WITHSELF-EXCITATION [72] Inventor: David D. Robb, Ames, Iowa [73] Assignee:Iowa State University Research Foundation, Inc., Ames, Iowa [22] Filed:Dec. 16, 1970 21] Appl. No.: 98,757

[52] US. Cl ..3l8/229, 318/418, 318/419,

[ 1 Feb. 29, 1972 OTHER PUBLICATIONS Hyde, Mai-bury, Solving a MotorStarting Voltage Problem, Westinghouse Engineer, May 1944, pp. 70- 73.

Primary Examiner-Gene Z. Rubinson Attorney-Dawson, Tilton, Fallon &Lungmus .[57 ABSTRACT I A polyphase induction machine has a balancedpolyphase shunt capacitor bank connected to the stator. In starting as amotor, the stator and capacitor bank are supplied with reduced linevoltage by means of a transformer, and the motor is allowed toaccelerate until it reaches maximum (and nearly) synchronous speed.Then, the motor is disconnected from the source and permitted to coast,whereupon there is a building up of stator voltage by self-excitation.When the phase and amplitude of the self-excited voltage areapproximately equal to the phase and amplitude of the line voltage, thestator is connected directly to the line and the transformer andexcitation capacitor are disconnected. The starting is accomplished withminimal dip in line voltage.

6 Claims, 11 Drawing Figures SYNCHRONIZING RELAY 1 36' svsrsm'ronSTARTING INDUCTION orons WITH SELF-EXCITATION BACKGROUND OF THEINVENTION 1. Field of the Invention The present invention relates toelectrical induction machinery. More particularly, it'relates to asystem, including apparatus and method, for starting induction motors,and it has particular advantages in starting-large polyphase inductionmotors under lightinitial loading.

Large induction motors frequently cannot be started by connecting thestator directly to'the supply line even if the motors are lightly loadedbecause to do so would cause .a large fluctuation or dip," as it issometimes called, in the line voltage because of the large transientcurrent that would result in a'direct connection.

2. Prior Systems Although not currently used in practice, one way tostart large induction motors would be to use a variable autotransformerwith thestator of the motor connected'to the variable terminal of theautotransformer, initially at a much reduced voltage. If the motor is athree-phase induction motor, a three? phase autotransformer is used.Since the present invention is primarily concerned with starting largeinduction motors and because the larger induction motors are ordinarilythreephase, the description of the invention will, for the most part,refer to three-phase induction motors, although persons skilled, in theart will appreciate that the invention is not so limited. In using avariable autotransformer, the motor is started at a lower voltage andthe applied terminal voltage is gradually increased to line voltage byvarying the autotransformer. Various devices such as current meters maybe used so that an operator does not permit the supply current to exceeda predetermined limit. In this case, the stator and line voltages arealways in phase and the stator and line currents arealways inphase.

A manually operated transformer-type starter that isused in practiceconsists of an autotransformer having a single reduced voltage tap foreach phase, switching to full voltage is done manually by the operatorat a time he feels is wise. Ordinarily suchmanual starters employ open.transition" switchingthat is, the stator is disconnected from thetransformer during the transition to full applied voltage. Since thisswitching is without synchronization, the transient current accompanyingthe switching may, by random chance, be larger than the motor startingcurrent at full voltage because the selfinduced stator voltage may be,in the worst case, 180 out of phase with the line voltage. Thus, theaccompanying line voltage fluctuation may be more severe than had themotor been started byadirect connection to line voltage. Further, verylarge transient torques, of the order of times the rated full load forthe motor, may result from the unsynchronized switching to full linevoltage.

It is desirable, of course, to have startup of large induction motors beindependent of an operators judgment, and systems are known for theautomatic starting of induction motors. One such system which-iscurrently supplied by manufacturers of motor control equipment issometimes referred to as the Korndorfer" motor starting method. In thismethod, the stator winding of each phase is connected to an intermediatetap of one'phase of a three-phaseautotransformer. The three phases ofthe autotransformer are connected by means of a contactor in a Y-array.The autotransformer is first connected across the new that the motor isstarted ata reduced voltage. After a predetermined time or conversely,afterthe stator current has reduced to a predetermined level,theautotransformers are disconnected by disconnecting'the Y-array sothat a portion of each phase of the autotransformer acts as a reactanceinseries with each stator winding. Finally, each statorwinding isdirectly connected to-line voltage, and the autotransformer isdisconnected. This is sometimes referred to as-a closed transition"starter systembecausethestator is always connected to thelinepeventhough the connection may be viaan autotransformer or a part of. anautotransformer windingacting as an inductor.

SUMMARY OF THE INVENTION In the present invention, a: balanced polyphasecapacitor bank is connected in parallel with the stator terminals, andthis capacitor is sometimes referred to as an excitation capacitor. Athree-phase autotransformer is first connected to the stator andcapacitor via a tap to start the motor at a reduced voltage. After apredetermined time has lapsed (sufficient to permit the motor to achieveits maximum speed for that applied voltage), themotor and capacitor aredisconnected from the autotransformer and the motor is permitted tocoast. The excitation capacitor provides a return path for the statormagnetizing current. The disconnection of the stator and capacitor fromthe tap of the autotransformer may occur, as mentioned, after a presettime delay or it may occur at a predetermined speed mat a predeterminedcurrent.

While thus coasting, theself-excited voltage in the stator of the motorwill increase while the speed of the motor reduces slightly. Anautomatic synchronizing relay sensing both the self-excited voltage ofthe motor as well as the line voltage then switches the motor directlyto the line when these voltages are equal in magnitude and phase. Whilecoasting, the frequency of the stator currents decreases to about 55cycles per second, which facilitates switching to full line voltage atthe proper phase and amplitude. Finally, the autotransformer andexcitation capacitors are disconnected from the system, and the motor isleft to run on full line voltage.

With this starting system when the motor is switched to full linevoltage, we have found that transients in stator current anddevelopedtorque are always well within the rated torque range. for themotor. In addition, the current and torque transients are not seriouslyaffected by rather large differences (of the order of 15) in theinstantaneous phase between line voltage and the stator voltageyfurther, the operation of the systemis not critically dependent uponvariations in line and stator voltage magnitudes at the time of thesynchronized switching.

The performance of the system hasbeen found to improve as the combinedinertia of the motor and load increases so that it is moreadvantageousin the starting of larger motors. That is, increased inertia reduces themaximum instantaneous transients in line current and developed torque.The maximum transient disturbances in the motor bus voltages are notsigniflcantly influenced by reasonable variations in phase differencesbetween line and stator voltage at the time of switchingnor invariations in the combined inertia of motor and load. The total coastingtime while awaiting the synchronized switching to line voltage is of theorder of 0.5 second. The preferred system is fully automatic, notrequiring judgment on the part of an operator.

Other features and advantages of the present invention will be apparentto persons skilled in the art from the following detailed description ofa preferredembodiment accompanied by'the attached drawings.

THE DRAWING starting an induction motor according to .the systems ofFIGS.

1 and 2; and

FIGSr4A-4Hrillustratevarious per unitcurrent, voltage andtorquerelationshipsduringa'startingcycle.

phase'starting system for an induction motor, the equivalent circuit forthe single phase for the motor being enclosed within a dashed line 10.The induction motor 10 includes a stator 11,

the equivalent circuit of which includes an inductance l2 and aresistance 13, and a rotor 14, the equivalent circuit of which includesan inductance l5 and a resistance 16. The output of the rotor 14 ismechanically coupled to a load schematically designated by referencenumeral 17.

The voltage bus or supply line for the single illustrated. phase isschematically illustrated at 20, and the equivalent circuit of thesource also includes an inductance 21 and a resistance 22.

A capacitor 24 is connected across the terminals of the stator 11 inseries with a first set of normally open contactor poles, designated S.An autotransformer 25 including a tap 250 has one terminal connected toground or system common and another terminal connected by means of asecond set of normally open contactor poles (also designated S) to thesupply bus. The supply bus is coupled by means of a normally opencontactor R to the stator of the motor 11. The tap 25a of theautotransformer 25 is connected by means of normally open contactorpoles T to the stator 11 of the motor 10.

In operation, the first step is to connect the transformer 25 across theappropriate phase of the stator 11 of the motor by closing both sets ofcontacts designated S as well as the contacts T. That is represented byStep 1 in the sequence of operations illustrated in the table of FIG. 3.Thus, the primary of the autotransformer 25 is connected across thesupply line and the capacitor 24 and stator 11 are connected in parallelto the tap 25a of the autotransformer 25. The motor is thus started at areduced voltage.

After the motor has come up to maximum speed and the stator current hasfallen to a low steady state level, the contacts T are opened to removethe motor stator and the capacitor from the autotransformer, and themotor is left to coast. While the motor is coasting, the self-excitedvoltage in the stator will build up and the stator current will increasewhile the motor speed decreases slightly. At this time, of course, thecontacts S are closed so that the capacitor 24 is connected across thestator 11 to provide a return path for stator current.

When the self-excited stator voltage reaches the same magnitude as theline voltage and the two are in phase, the contacts R are closed toconnect the stator directly to the line (Step 3 of FIG. 3). This is thesynchronized switching of the coasting motor directly to the line.Finally, the sets of contacts S are opened to remove the autotransformer25 as well as the capacitor 24 from the line (Step 4 of FIG. 3), and thestarting operation is thus completed.

We have found that variations in the phase angle at the time ofswitching between the line voltage and the stator voltage of up to about:15" do not substantially affect operation. Further, differences of theorder of 15 percent between the amplitude of the line voltage and theamplitude of the selfexcited stator voltage do not appreciably affectoperation. However, in practice, synchronized switching at exact linevoltage and phase would not be difficult to achieve because thefrequency of the stator voltages drops to about 55 Hz. whereas the linefrequency remains at 60 Hz.

Although the illustration of FIG. 1 shows that the excitation capacitor24 is connected directly across the winding for the illustrated phase ofthe motor, persons skilled in the art will appreciate that the inventionis not so limited. That is, the polyphase motor windings may beconnected in a A or in a Y- arrangement, and similarly, the capacitorbank may be connected in a A or a Y-arrangement independent of themanner in which the stator windings are connected. Thus, the capacitorbank may be connected in a A whereas the motor stator windings may beconnected in a Y. It is only necessary that the capacitor bank beconnected so as to permit the stator currents to circulate during thetime when the motor is disconnected from the line. Ordinarily, this willtake the form of a balanced polyphase capacitor bank connected in shuntwith the stator winding.

Turning now to FIG. 2, there is shown a more detailed schematic diagramfor starting an induction motor, again, only one phase of the motor andits associated control circuitry being shown. The single illustratedphase of the motor is generally designated by reference numeral 10. Theexcitation capacitor 24, autotransformer 25, and the contactors R, S andT are the same as shown in FIG. 1.

The contactor R is normally open, and it is in series with one phase ofthe line voltage. The contactor R is controlled by a relay designated 30which also actuates contacts R1, R2, R3, and R4 in FIG. 2. The contactsR1 are normally open, and they are connected in series with one section32 of a normally closed STOP switch and the coil of relay 30 across acontrol bus including lines 33 and 34. The first section of the STOPswitch 32 is mechanically ganged with a second normally closed sectionof that switch designated 35. Also actuated by means of the relay 30 aretwo sets of normally closed contacts designated R3 and R4 which areconnected to points respec tively on either side of the contactor R andleading to the input of a synchronizing relay 36 having a balanced inputand actuating normally open contacts SY which are connected parallelwith the contacts R1. The relay 30 also controls normally closedcontacts R2 which are connected in series with the second section of theSTOP switch 35, a normally open START switch 37 and the coils of threerelays (designated 38, 39, and 40) across the control bus 33, 34. Thecoils of the relays 38, 39 and 40 are connected in parallel. The relay38 is a time delay relay of the type which actuates its contacts onlyafter a preset and predetermined time after the coil of that relay hasbeen energized. The relay 39 controls the actuation of the contactors Sas well as the normally open contacts S1. The relay 40 controlsoperation of the contactor T. Further, as schematically illustrated bythe chain line 41, the contactors R and S are mechanically interlockedsuch that the contactor S opens after the contactor R closes.

The time delay relay 38 actuates a set of normally closed contacts TDlconnected in series with the coil of relay 40 as well as a pair of setsof normally opened contacts TD2 and TD3 connected in series respectivelywith the contacts R4 and R3 in the input lead to the synchronizing relay36.

Although the illustrated control system was manually operated, momentarycontact START and STOP pushbuttons, persons skilled in the art willappreciate that other devices may be substituted for these pushbuttonssuch as pressure switches, liquid level switches, thermostats, etc.,which will perform equivalent functions and thereby render the systemfully automatic.

In operation of the system of FIG. 2, an operator first pushes the STARTpushbutton 37 which will cause all of the coils of the relays 38, 39 and40 to be energized by the control bus 33, 34 since the contacts R2 andTlDll are normally closed. Immediately thereafter relay 39 closescontacts S1 to electrically lock in the coils of relays 38, 39 and 40.The relay 39 also closes the contactors 8,5. The closing of thesecontactors connects the primary of the autotransformer 25 across thesupply line and also connects the excitation capacitor 24 to the statorof the motor 10. At the same time, the relay 40 closes the contactors Twhich connects the parallel combination of the excitation capacitor 24and motor stator to the secondary tap of the autotransformer 25 therebyenergizing the motor at a reduced voltage. The motor will begin toaccelerate and within a known time (depending upon the load) the motorwill achieve maximum speed. At that time, as determined by the timedelay relay 38 the contacts TDI open to deenergize the coil of relay 40,and the contactors T thereupon open. Thus, the system is at step 2 inthe sequence of FIG. 3. Further, however, the synchronizing relay 36 hasone input connected to the supply line by means of contacts TD2 and theother input connected to the motor stator by contacts TD3, although thesynchronizing relay 36 is not energized at this time. The

synchronizing relay 36 may be of the type commercially available as TypeXA Automatic Synchronizer and Voltage Acceptor, Style No. 127D74l G4manufactured by Westinghouse Electric Corporation. The function of thesynchronizing relay 36, of course, is to close contacts SY when theamplitudeand phase of a supply line voltage and the stator voltageforthemotor are within predetermined limits. As

already mentioned, when the motor is disconnected from theautotransformer 25 by opening contacts T, the self-excited statorvoltage will build up and the angular velocity of the motor vwill reduceslightly so that the frequency of the stator voltage reduces from 60 Hz.to 55 Hz. Operation of the system is now at step 3 of the sequence ofFIG. 3.

When the synchronizing relay 36 senses that-the phase and amplitude ofthe self-excited stator voltage is within the predetermined limits ofthe supply voltage, it will actuate the contacts SY thereby causing thecoil of relay 30 to become energized through the normally closedpushbutton STOP switch section 32. When the relay 30 is energized, thecontacts R1 are closed to lock in the coil of relay 30, and, at the sametime, contacts R2, R3 and R4 are opened to disconnect the input of thesynchronizing relay 36 and to deenergize the coils of relays 38 and 39.The coil of relay 40 will already have been deenergized by opening ofthe contacts TDl. The relay 30 also closes the contactor R whichconnects the motor stator directly to the supply line. When thecontactor R opens, the previously mentioned interlock causes thecontactors 8,8 to open and the system thereafter operates according tostep 4 of the sequence of FIG. 3. An operator may shut down the systemby pressing the STOP pushbutton.

In the embodiment illustrated in FIG. 2, the disconnecting of the: motorfrom the tap of the autotransformer 25 occurs after a predeterminedvlapse of time caused by the time delay relay38. This time delay, as hasalready been mentioned, is sufficient to insure that the motor hasachieved maximum speed-that is, the synchronous speed of the motor lessslip speed. However, this disconnecting of the motor from the tap of theautotransformer may also be occasioned by sensing line current or statorcurrent. For example, a current-relay sensing line current or statorcurrent would become energized after the sensed current had fallen to apredetermined nns value thereby indicating that the motor has achievedmaximum speed. Further, this disconnecting of the motor from theautotransformer tap could also be achieved by means of a device, such atachometer, sensing motor speed and generating a signal when the motorhas reached maximum speed to disconnect the stator from theautotransformer.

Turning now to the diagrams of FIGS. 4A-I-l, the vertical represents perunit or normalized values of the various system parameters, and thehorizontal axis represents time. The graphs are arranged so that time iscommon to each set of graphs. Both graphs (i.e., left and rightsections) represent an instantaneous phase difference at the time ofswitching between ,line and the self-excited voltage of 5 electricaldegrees. The combined inertia constant of motor and load is 0.67-perunit for the left-hand set of graphs, and the right-hand set of graphsare for a combined inertia constant of 1.50 per unit. Graph. 4Arepresents one phase of line current (i Graphs 4B-4D represent absolutevalues of the line-to-neutral voltages. Graph 4E is the selfrexcitedstator voltage for .one phase. Graph 4F shows one phase of statorcurrent. Graph 46 shows developed torque; and graphAI-l shows the perunit motor speed, i.e., the ratio of rotor angular velocity to thesynchronous speed.

As seen in graph 4G, reference numeral 35 indicates the time at whichthe motor and excitation capacitor 24 are disconnected from theautotransformer 25 by opening the contacts T. At this time, the per unitspeed (which is also the ratio of the angular velocity of the rotor tothe radian frequency of the distribution system) is close to synchronousspeed. The per unit stator current (represented by FIG. 4F) has reacheda steady state, as has the per unit. stator voltage (FIG. 4E).

As seen in graph 4H, as the motor coasts, the per unit speed reducesslightly, whereas the per unit stator voltage (FIG. 45) increases asdoes the per unit stator current (FIG. 4F). During coasting, of course,there is no line current (FIG. 4A). At the time that the stator isswitched to the line voltage as indicated by reference numeral 36 inFIG. 46, the line voltage and the stator voltage are in phase and ofequal magnitude. There are slight fluctuations in the'line voltage(FIGS. 48, 4C, and 4D show the line voltage fluctuations) and there is acorresponding increase in the motor torque (FIG. 4G). The line currentincreases, and gradually,the per unit speed increases back to normal.However, it will be appreciated that the line voltage fluctuation isminimal and that the increase in line current is substantially reduced.

Having thus described in detail a preferred embodiment of the invention,persons skilled in the art will be able to substitute equivalentelements for those which have been disclosed and to otherwise modify theillustrated system while continuing to practice the principle of theinvention and it is, therefore, intended that all such modifications andequivalents be covered as they are embraced within the spirit and scopeof the appended claims.

I claim:

1. A system for starting a polyphase induction motor having a stator anda rotor comprising: voltage transformer means for connecting the statorof said motor to line voltage at a reduced voltage, a polyphaseexcitation capacitor bank connected in parallel with said stator of saidmotor, first circuit means associated with said motor for disconnectingsaid motor and said capacitor bank from said voltage transformer meansafter said motor has come to speed under said reduced voltage,synchronizing switching means for switching said stator and capacitorbank directly to said line when the amplitude and phase of theself-excited voltage of said stator has come within a predeterminedlimit relative to said line voltage, and second circuit means forthereafter disconnecting the transformer means and said capacitor bankfrom said motor and said line.

2. The system of claim 1 whereinsaid first circuit means includes presettime delay means for disconnecting said stator and said capacitor bankfrom said voltage transformer means a predetermined time after saidstator has been initially energized whereupon the line current will havefallen to a steady state value and the speed of said motor will havereached a maximum value.

3. The system of claim 1 wherein said first circuit means includescurrent sensing means for disconnecting said stator and said capacitorbank from said voltage transformer means when the line current falls toa predetermined value.

4. The system of claim I wherein said second circuit means comprisesmeans for sensing the speed of said motor for disconnecting said statorand said capacitor bank from said voltage transformer means when thespeed of said motor has reached a predetermined value.

5. The system of claim 1 wherein said synchronizing switching meansincludes an automatic synchronizing relay having a pair of inputterminals; thirdcircuit means responsive to said time-delay means forconnecting said input terminals of said synchronizing relay respectivelyto said line voltage and to said. stator voltage when said stator isdisconnected from said line voltage; and fourth circuit means responsiveto said synchronizing relay for connecting said stator to said line whenthe amplitudes of the self-excited stator voltage and said line voltageand the phases of said stator voltage and said line voltage are withinpredetermined tolerance limits.

6. A 'method of starting a large induction motor having a polyphasestator and rotor comprising: providing a polyphase excitation capacitorbank connected in parallel with the motor stator, connecting each phaseof said stator to a reduced line voltage, then permitting themotor toreach maximumspeed, then disconnecting said stator and capacitor bankfrom said reduced voltage and permittingsaid motor to coast, the currentin said'stator circulating through said excitation capacitor bank, thenswitchingsaidstator directly to said line when the magnitude and phaseof the self-excited stator voltage are within predetermined limits ofthe magnitude and phase respectively of the line voltage.

1. A system for starting a polyphase induction motor having a stator and a rotor comprising: voltage transformer means for connecting the stator of said motor to line voltage at a reduced voltage, a polyphase excitation capacitor bank connected in parallel with said stator of said moTor, first circuit means associated with said motor for disconnecting said motor and said capacitor bank from said voltage transformer means after said motor has come to speed under said reduced voltage, synchronizing switching means for switching said stator and capacitor bank directly to said line when the amplitude and phase of the selfexcited voltage of said stator has come within a predetermined limit relative to said line voltage, and second circuit means for thereafter disconnecting the transformer means and said capacitor bank from said motor and said line.
 2. The system of claim 1 wherein said first circuit means includes preset time delay means for disconnecting said stator and said capacitor bank from said voltage transformer means a predetermined time after said stator has been initially energized whereupon the line current will have fallen to a steady state value and the speed of said motor will have reached a maximum value.
 3. The system of claim 1 wherein said first circuit means includes current sensing means for disconnecting said stator and said capacitor bank from said voltage transformer means when the line current falls to a predetermined value.
 4. The system of claim 1 wherein said second circuit means comprises means for sensing the speed of said motor for disconnecting said stator and said capacitor bank from said voltage transformer means when the speed of said motor has reached a predetermined value.
 5. The system of claim 1 wherein said synchronizing switching means includes an automatic synchronizing relay having a pair of input terminals; third circuit means responsive to said time delay means for connecting said input terminals of said synchronizing relay respectively to said line voltage and to said stator voltage when said stator is disconnected from said line voltage; and fourth circuit means responsive to said synchronizing relay for connecting said stator to said line when the amplitudes of the self-excited stator voltage and said line voltage and the phases of said stator voltage and said line voltage are within predetermined tolerance limits.
 6. A method of starting a large induction motor having a polyphase stator and rotor comprising: providing a polyphase excitation capacitor bank connected in parallel with the motor stator, connecting each phase of said stator to a reduced line voltage, then permitting the motor to reach maximum speed, then disconnecting said stator and capacitor bank from said reduced voltage and permitting said motor to coast, the current in said stator circulating through said excitation capacitor bank, then switching said stator directly to said line when the magnitude and phase of the self-excited stator voltage are within predetermined limits of the magnitude and phase respectively of the line voltage. 